1. Field of the Invention
This invention relates to a vehicle electromagnetic clutch control device which can be miniaturized with its mounting substrate miniaturized.
2. Description of the Prior Art
A vehicle electromagnetic clutch control device of this type has been disclosed by Japanese Patent Application (OPI) No. 31533/1987 (the term "OPI" as used herein means an "unexamined published application"). FIG. 3 is a circuit diagram showing the arrangement of the conventional vehicle electromagnetic clutch control device.
In FIG. 3, reference numeral 101 designates a microcomputer for providing a clutch current instruction signal I.sub.s ' according to an engine control data S.sub.E and travel control data S.sub.D ; and 800, a D/A (digital-to-analog) converter for converting a digital signal outputted by the microcomputer 101 into an analog signal to obtain an analog current instruction signal I.sub.S.
The current applied to an electromagnetic clutch 502 is controlled according to the analog current instruction signal provided by the D/A converter 800, thereby to control the torque transmitted through the clutch.
The microcomputer 101 outputs a plurality of digital signals, which are applied to digital signal buffers 803 through 808 in the D/A converter 800, where they are weighted, respectively. The output signals of those digital signal buffers are applied to a D/A conversion resistor circuit 802, where they are converted into analog signals.
The digital signal buffer amplifiers 803 through 808 are employed to cause the logical output voltage of the microcomputer to reach the ideal logical output voltage as much as possible, because it is difficult to obtain the supply voltage when the logical output voltage of the microcomputer 101 is at "H" level, and it is also difficult to obtain the ground voltage when it is at "L" level. The digital signal buffer amplifiers 803 through 808 are such that internal voltage drop is ranged from 50 mV to 100 mV.
The output of the D/A conversion resistor circuit 802 is applied to the positive (+) input terminal of an analog buffer amplifier 801, the negative (-) input terminal of which is connected to the output terminal thereof.
The current instruction signal I.sub.S provided at the output terminal of the analog buffer amplifier 801 is applied to the positive (+) of a PWM (pulse width modulation) comparator, to the negative (-) of which a current feedback signal I.sub.F provided at the output terminal of a current detecting amplifier 301 is applied.
The PWM comparator 201 subjects the current instruction signal I.sub.S and the current feedback signal I.sub.F to comparison, and outputs an on/off signal for a PWM control output transistor 406 according to the difference between those signals.
The output of the PWM comparator 201 is used to turn on and off a signal converting transistor 403. The base of the signal converting transistor 403 is grounded through a resistor 402 and connected through resistors 401 and 202 to the power source. The emitter of the transistor 403 is grounded, and the collector thereof is connected through resistors 404 and 405 to the power source.
The connecting point of the resistors 404 and 405 is connected to the base of the PWM control output transistor 406. The emitter of the transistor 406 is connected to the power source, and the collector thereof is grounded through a circulation diode 501, and connected to an output terminal 503.
The aforementioned electromagnetic clutch 502 is connected between the output terminal 503 and another output terminal 504. The electromagnetic clutch 502 comprises a clutch coil 502a, and slip rings 502b and 502c.
The output terminal 504 is connected to the collector of a quick-break transistor 601. A constant voltage diode 602 is connected between the collector and the base of the transistor 601, the emitter of which is grounded through a output current detecting resistor 701.
One of the terminals of the output current detecting resistor 701 is connected through a resistor 302 to the positive (+) input terminal of the current detecting amplifier 301, and the other terminal of the output current detecting resistor 701 is connected through a resistor 303 to the negative (-) input terminal of the current detecting amplifier 301. The negative (-) input terminal of the amplifier 301 is connected through a resistor 304 to the output terminal thereof. The above-described current feedback signal I.sub.F is provided at the output terminal of the current detecting amplifier 301.
The microcomputer 101 outputs a clutch release signal, which is applied through a resistor 609 to the base of a signal converting transistor 607. The base of the transistor 607 is grounded through a resistor 608, and the collector thereof is connected through a series circuit of resistors 605 and 606 to the power source.
The connecting point of the resistors 605 and 606 is connected to the base of a signal converting transistor 604. The emitter of the transistor 604 is connected to the power source, and the collector thereof is connected through a resistor 603 to the base of the aforementioned quick-break transistor 601.
The operation of the vehicle electromagnetic clutch control device thus organized will be described. As was described above, the microcomputer 101 provides a clutch current instruction signal according to engine control data S.sub.E and the travel control data S.sub.D, and outputs a digital current instruction signal.
The digital current instruction signal is applied to the D/A converter 800, where it is converted into an analog current instruction signal I.sub.S. The analog current instruction signal I.sub.S is applied to the PWM comparator 201, where it is compared with the current feedback signal I.sub.F outputted by the current detecting amplifier 301 and the difference signal is subjected to pulse with modulation, so that the signal converting transistor 403 is turned on and off according to the pulse width of the resultant signal.
As the signal converting transistor 403 is turned on and off in this manner, the PWM control output transistor 406 is also turned on and off, to control the flow of a clutch current I.sub.C to the electromagnetic clutch 502; that is, the clutch current is allowed to flow into the clutch or not.
On the other hand, the quick-break transistor 601 is controlled by the clutch release signal outputted by the microcomputer 101 so that it is turned on for the normal connection of the clutch. That is, the signal converting transistors 607 and 604 are turned on by the clutch release signal, and therefore the quick-break transistor 601 is also turned on. Hence, when the PWM control output transistor 406 is turned on by the difference signal pulse-width-modulated by the PWM comparator 201, the clutch current is allowed to flow from the power source into a closed circuit of the PWM control output transistor 406, the output terminal 503, the electromagnetic clutch 502, the output terminal 504, the quick-break transistor 601, the current detecting resistor 701, and the ground.
A voltage drop is developed across the current detecting resistor 701 according to the clutch current I.sub.C. The potentials at both ends of the resistor 701 due to the voltage drop are applied through the resistors 302 and 303 to the positive (+) input terminal and the negative (-) input terminal of the current detecting amplifier 301, so that the latter 301 outputs the current feedback signal I.sub.F.
Thus, in accordance with the pulse width modulation signal of the difference signal outputted by the PWM comparator 201, the clutch current flows in the electromagnetic clutch 502 to operate the latter. The electromagnetic clutch 502 is released by the clutch release signal provided by the microcomputer 101.
That is, the quick-break transistor 601 is normally held tuned on, and the release of the clutch is controlled by the clutch release signal. The clutch release signal is outputted by the microcomputer 101, and applied through the resistor 609 to the base of the signal converting transistor 607, to turn on the latter 607. As a result, the collector potential of the transistor 607 is decreased, so that the signal converting transistor 604 is turned off.
When the signal converting transistor 604 is turned off in this manner, the quick-break transistor 601 is turned off, thus interrupting the flow of the clutch current to the electromagnetic clutch 501. That is, the electromagnetic clutch 501 is released.
The conventional vehicle electromagnetic clutch control device is designed as described above. Therefore, in order to improve the resolution of the current instruction signal, it is necessary to increase the number of digital signal buffer amplifiers (803 through 808), the number of resistors forming the D/A conversion resistor circuit 801, and the number of output terminals of the microcomputer. For this purpose, it is necessary to increase the size of a substrate for mounting the device, which obstructs miniaturization of the device itself.